Kemen Uranga Artola, 43, was arrested in the north of London on a European arrest warrant.
He was to appear in court later for a hearing over his extradition to Spain, London police said.
He is the fourth ETA suspect arrested in Britain in recent months, and the 19th arrested overall this year, the Spanish ministry said.
Uranga has been sought by Spain since 2001.
Media reports say Uranga is suspected of the murder of a judge, Jose Maria Lidon, who was killed by an ETA group in 2001.
ETA said last year it would give up its armed struggle, which lasted more than 40 years and claimed more than 800 lives. (M014)
Editor: Kunto Wibisono
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